Control of heatable discharge lamps



Jan. 3, 1967 R. TAYLOR CONTROL OF HEATABLE DISCHARGE LAMPS Filed Dec.15, 1963 United States Patent 3,296,488 CONTROL OF HEATABLE DISCHARGELAMPS Raymond Taylor, Birstafl, England, assignor to AssociatedElectrical Industries Limited, London, England, a British company FiledDec. 13, 1963, Ser. No. 330,432 1 Claim. (Cl. 315-116) This inventionrelates to the control of heatable discharge lamps, and aims atproviding an improvement which makes it possible to maintain accuratelythe degree of brightness as is set by an input power control.

It has been known that discharge lamps, such as lowpressure sodiumlamps, can be operated with greatly reduced power input in order to dimtheir brightness. even to less than 1% of their maximum light output,provided the vapour pressure in the lamp envelope is maintained such asby external heating. Usually an outer envelope of transparent materialis provided to accommodate the lamp and an electric resistance heater.Such lamps are commonly known as oven lamps.

If the lamps are normally supplied from an AC. source, controlledoppositely poled, parallel connected rectifiers can serve for reducingthe power input, while avoiding start failures due to insuflicientstriking voltage, when the rectifiers are made conducting at a point ofa sinusoidal voltage near zero voltage.

The actual light output from a gaseous discharge lamp depends on boththe lamp efliciency and current magnitude. It has been found that thelamp efiiciency changes with the shape of the actual current wave, whichdepends upon the supply voltage, and the invention aims at overcomingthis difiiculty, particularly where accurate dimming, say to 3%, and 30%of maximum brightness is required, as for visual light approachindicator systems on aerodromes.

Accordingly the present invention resides in an arrangement forsupplying current to an electric discharge lamp from an AC. sourcecomprising parallel connected oppositely poled rectifiers, switch meansfor inserting into or removing from the path between said source andlamp said rectifiers, also comprising a means for controlling theconducting periods of the rectifiers, and a voltage regulatormaintaining constant the input voltage of said rectifiers.

In the accompanying drawings:

FIGS. 1 and 2 show how the shape of a rectified current impulse dependsupon the voltage wave form of the alternating current from which it isderived; and

FIG. 3 shows diagrammatically and by way of example a circuitarrangement according to the invention.

Referring to FIG. 1, it is assumed that oppositely poled rectifiersbecome conductive at instants A1 and B1 during the positive and negativehalf cycles. For a given voltage wave v1, the current pulses are shapedas indicated at 01, 02. If the AC. voltage amplitude is reduced to theshape v2 of FIG. 2, a similar power input and mean current can beobtained by rendering the rectifiers conductive at instants A2, B2,nearer the wave peaks. However, the shape of the current waves 03, c4 isdifierent from that of 01, 02, therefore the lamp efliciency changesPatented Jan. 3, 1967 and the light output of the lamp differs. Thus inspite of a consistent watt input the lumen output varies. The inventionavoids this drawback and insures a consistent light regulationcharacteristic.

Referring to FIG. 3, a sodium discharge lamp 1 is supplied throughterminals 3, 4, from a single-phase source 2. An envelope 5 oftransparent material contains the lamp and a heating resistor 6, thusrepresents the oven. Oppositely poled rectifiers 7, 8 are connectedthrough a voltage regulator 9 to the terminals 3, 4. A change-overswitch 10 is provided to connect the lamp either to the terminals 3, 4for maximum light output, or to the rectifier output for dimmedlighting. The rectifiers 7, 8 have control electrodes 11, 12 suppliedfrom a control box 13 which provides in a conventional manner lamptrigger impulses. It also includes a means for regulating the amount oflamp heating, which increases as the input to the lamp decreases. Thelight output can be reliably controlled if the rectifier input voltageis maintained constant and the vapor pressure in the lamp is not allowedto drop unduly with the lamp current reduction. According to theinvention as this vapor pressure tends to decrease, due to reduction ofthe rectifier conducting period, the external lamp heating isautomatically increased, so as to achieve a desired dimming etfect. Thusa manual member 14 is provided to set at will the lamp input andadequate heating input for a desired degree of lamp brightness.

Modifications are possible without departing from the invention asdefined by the appended claims. The regulator 9 can be any type known inthe art to ensure an output voltage constant to a desired degree whilethe input voltage may vary within a predetermined range. Thyratrons andsemiconductor devices can be readily controlled by triggering voltage orcurrent pulses and can supply adequate power outputs. A gradual andcontinuous control, or settable discrete steps such as 3%, 10% or 30%,of a full rating can be provided. The switch 10 may be remotelycontrolled by means of relays, for instance.

What I claim is:

In the combination of an electric discharge lamp and means forcontrolling the brightness thereof, the improvement comprising parallelconnected oppositely poled rectifiers, an AC. source, switch means forselectively con necting said rectifiers in the circuit between saidsource and said lamp, a voltage regulator to maintain constant inputvoltage to said rectifiers, an electric heater for the lamp and controlmeans for controlling the conducting periods of said rectifiers and forincreasing the heater input as the conducting periods of the rectifiersare reduced.

References Cited by the Examiner UNITED STATES PATENTS 2,765,416 10/1956Beese et a1. 3l315 X 3,205,404 9/1965 Kurata et a1 315-194 JAMES W.LAWRENCE, Primary Examiner.

S. SCHNEEBERGER, Assistant Examiner.

